1. Field of the Invention
The present invention relates to a fuel cell system with a catalytically active reactor unit for at least partial chemical transformation of an operating medium stream, especially a reformer, gas purifier stage and/or a combustor, in which the catalytically active reactor unit has a catalytically active reactor volume acted on by the operating medium stream.
2. Description of the Related Art
Fuel cell technology is becoming ever more important, especially in connection with consumer-driven concepts for vehicles. Fuel cells offer the possibility to convert chemical energy directly to electrical energy, which subsequently can be converted into mechanical drive energy with the aid of an electrical motor.
Because of technical problems with the hydrogen supply in vehicles the hydrogen must be produced, as needed, e.g. by a so-called reforming unit or partial oxidation of hydrocarbons, especially by means of a catalytic reactor and/or reformer. This sort of hydrocarbon material is usually present in the form of a commercial fuel, such as gasoline or diesel fuel, however other hydrocarbon materials, for example natural gas, methane, methanol, liquified petroleum gas (LPG) or synthetic fuels, can be used for this purpose.
The reformer usually has a catalytically coated surface, especially on a porous catalyst structure with numerous channels or the like extending in the direction of flow or the fuel stream. This catalytically active surface is acted on generally with a mixture of fuel, water and/or air.
So-called PEM fuel cells are frequently used in commercial fuel cell systems, which however react to the carbon monoxide content in a hydrogen-rich medium with a “contaminated appearance” on the catalytic anode. Thus the conversion of hydrogen at the electrode is made more difficult or prevented. For this reason suitable fuel cell systems must guarantee the production of a hydrogen-enriched medium, which is as most free of carbon monoxide.
Thus the carbon monoxide content in hydrogen-enriched reformate has already been nearly completely reduced with the help of additional catalytic reactors. For example, in a first step for this purpose one reactor unit is connected downstream to the reformer, which oxidizes the carbon monoxide resulting from reforming the fuel to form CO2 by addition of water by means of a so-called “shift reaction”. In this “shift reaction” additional hydrogen is released. However a residue of carbon monoxide remains in the reformate gas in a concentration, which always still leads to an intolerable contamination of the fuel cell.
Additional catalytically active reactors are used, as needed, to convert the still present carbon monoxide residue, which up to now reduce the carbon monoxide residual amount nearly completely by catalytic oxidation of the remaining carbon monoxide with added oxygen in a suitable catalytic oxidation unit. In order to reduce the carbon monoxide content to a value less than 50 ppm, preferably carbon monoxide multi-stage oxidation units are used, in which oxygen is supplied separately for example to each stage. The oxygen is metered or delivered for this purpose generally in the form of air oxygen.
Furthermore additional catalytically active reactor units, for example, for methanation of reformate gas, are used in fuel cell systems alternatively or in combination with the above-described reactor units. In methanation of reformate gas hydrogen and carbon monoxide are transformed chemically to methane so that the carbon monoxide content of the reformate gas is lowered further.
Furthermore catalytically active combustors are used to produce heat in commercial fuel cell systems. For example a residual anode gas stream, a reformate gas stream of poor quality, which is produced in a starting stage or the like with comparatively low hydrogen content, is catalytically converted in a suitable combustor. Heat is produced thereby, which can be used, for example, for heating the most different parts of the fuel cell system.
Above all it is especially disadvantageous that the heat losses to the surroundings are comparatively high in current reformers in partial load operation, i.e. during throughput of comparatively small mass flows, so that they must compensate by supplying additional heat, in order to maintain the desired reformation temperature. This definitely leads to a reduced total efficiency for reforming during partial load operation. Furthermore the comparatively small flow speeds in partial load operation are disadvantageous, since above all the delay time during load change is long because of that. Also for the same reason the dynamic behavior of the entire system and/or the fuel cell system is decisively impaired.